Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gearshift mechanism connected to the torque converter, and a plurality of friction element actuated by hydraulic pressure for selecting one of the gear stages of the gearshift mechanism.
A hydraulic control system for the automatic transmission operates by the selective supply of hydraulic pressure, the flow of which is generated by a hydraulic pump, to each friction element (for engagement or disengagement of the same) by a plurality of control valves such that shifting can be realized automatically and appropriate to the driving state of the vehicle.
Such hydraulic control systems generally comprise pressure regulating means for controlling the hydraulic pressure generated by the hydraulic pump, manual and automatic shift control means for selecting a shift mode, hydraulic pressure control means for controlling shift quality and shift response, a damper clutch control means for actuating a damper clutch of the torque converter, and hydraulic pressure distributing means for supplying an appropriate amount of hydraulic pressure to each of the friction elements.
Referring to FIG. 6, there is shown a hydraulic circuit diagram of a conventional hydraulic control system. As shown in the drawing, the hydraulic control system includes a torque converter 2 connected to an output shaft of an engine and which converts and multiplies engine power and transmits the same to a transmission, and a hydraulic pump 4 which generates oil flow to create hydraulic pressure for shift stage control.
Connected through lines to comprise the pressure regulating means and the damper clutch control means are a pressure regulator valve 8 for regulating pressure to a predetermined level, a torque converter control valve 10 for controlling pressure to a predetermined pressure level for the torque converter 2 and for lubrication, and a damper clutch control valve 12 for improving power transmission efficiency of the torque converter 2.
Part of the hydraulic pressure generated by the hydraulic pump 4 is received by a reducing valve 14 which reduces hydraulic pressure flowing therethrough to a level lower than line pressure, and by a manual valve 16 indexed with a selector lever operated by the user.
Part of the hydraulic pressure reduced to a predetermined level by the reducing valve 14 is supplied to a high-low pressure valve 18 which reduces line pressure in high speed ranges to minimize damage to the hydraulic pump 4, and part of the hydraulic pressure is supplied to first and second pressure control valves 20 and 22, the high-low pressure valve 18 and the first and second pressure control valves 20 and 22 comprising the hydraulic pressure control means.
Further, part of the pressure supplied to the first and second pressure control valves 20 and 22 is supplied to an N-R control valve 24 to be used as control pressure for the same, the N-R control valve 24 reducing shift shock when shifting from a neutral N range to a reverse R range.
The manual valve 16 communicates with a shift control valve 28, changing hydraulic pressure lines by operation of first and second solenoid valves S1 and S2, via a line 26, through which hydraulic pressure flows when in a drive D range. The manual valve 16 and the shift control valve 28 comprise the manual and automatic shift control means.
Second, third, and fourth speed lines 30, 32, and 34 are connected to the shift control valve 28 to supply hydraulic pressure to each valve of the hydraulic pressure distributing means for shift stage control. Namely, the second speed line 30 supplies hydraulic pressure to a left-side port of a 1-2 shift valve 36 to control the same, the third speed line 32 supplies hydraulic pressure to a left-side port of a 2-3/4-3 shift valve 38 to control the same, and the fourth speed line 34 supplies hydraulic pressure both to a right-side port of the 2-3/4-3 shift valve 38 to control the same and to a left-side port of a 2-4/3-4 shift valve 40 to control the same.
The first pressure control valve 20 realizes port conversion by operation of a third solenoid valve S3, while the second pressure control valve 22 realizes port conversion by operation of a fourth solenoid valve S4.
A first speed line 44 is branched off from the line 26 connected to the manual valve 16 to supply hydraulic pressure to the first and second pressure control valves 20 and 22, and to a rear clutch C1, or a first friction element acting as a first speed input element, after passing through the 2-4/3-4 shift valve 40 by control of the third and fourth solenoid valves S3 and S4.
A timing control line L is connected to the first speed line 44 such that line pressure flowing therethrough can be supplied to a timing control valve 42.
The shift timing control valve 42 either supplies or releases operational pressure to or from an end clutch C3, operating as an input element in third and fourth speeds, and is able to adjust the timing between the supply of hydraulic pressure to the end clutch C3 and control of a kickdown servo C2, operating as a reaction element in second and fourth speeds. Namely, the shift timing control valve 42, when a fifth solenoid valve S5 is controlled to OFF by the TCU, either supplies operational pressure to the kickdown servo C2 or to the end clutch C3.
Further, hydraulic pressure is able to be supplied from the first pressure control valve 20 to the kickdown servo C2, operating as a reaction element in the second speed, via the 1-2 shift valve 36.
Part of the above hydraulic pressure passing through the 1-2 shift valve 36 is fed to the end clutch C3, operating as an input element in the third speed, via the 2-3/4-3 shift valve 38 and the shift timing control valve 42. Here, the hydraulic pressure supplied to the end clutch C3 is also supplied to a release side chamber hi of the kickdown servo C2.
When the manual valve 16 is in the reverse R range, hydraulic pressure fed to a reverse first control line 46 is supplied to a low-reverse brake C4, operating as a reaction element in the reverse R range, via the 1-2 shift valve 36. The manual valve 16 is connected to a reverse second control line 48 for operation of a front clutch C5, operating as an input element.
A check valve 50 is disposed on the reverse second control line 48, the check valve 50 delaying the release of hydraulic pressure to improve shift quality.
A kickdown switch 52 is disposed on an operational side chamber h2 of the kickdown servo C2. The kickdown switch 52 is controlled to OFF when hydraulic pressure is being supplied to the operational side chamber h2, and controlled to ON when hydraulic pressure is being supplied to the release side chamber hi, a signal of the ON or OFF state of the kickdown switch 52 being transmitted to the TCU.
To allow skip shifting by converting operational pressure of the kickdown servo C2 according to shift stage, the 2-4/3-4 shift valve 40 is provided with ports such that hydraulic pressure from the second pressure control valve 22 passes through the 2-4/3-4 shift valve 40, is fed to the timing control valve 42, then supplied to the kickdown servo C2.
Reference numeral S6 in the drawing denotes a sixth solenoid valve which controls the damper clutch control valve 12 to operate or disengage the damper clutch of the torque converter 2.
In the hydraulic control system structured as in the above, during 4-3 downshifting in a power ON state, pressure of the end clutch C3 is controlled to communicate with the release side chamber h1 of the kickdown servo C2 in a state where the pressure level in the operational side chamber h2 is maintained.
That is, pressure of the end clutch C3 is initially maintained at a high level then slowly reduced by low duty control of the third solenoid valve S3. At the same time, a valve spool of the 2-3/4-3 shift valve 38 is moved to the right (in the drawing) by the release of fourth speed pressure, realized by control of the shift control valve 26, such that part of the hydraulic pressure supplied to the end clutch C3 is fed to the release side chamber h1 to disengage the kickdown servo C2.
However, as shown in FIGS. 6 and 7, resulting from the formation of an orifice (ori) on a live leading into the 2-3/4-3 shift valve 38, pressure buildup in the release side chamber h1 of the kickdown servo C2 is delayed. Accordingly, the level of pressure supplied to the end clutch C3, connected to the release side chamber h1 through the shift timing valve 42, is temporarily reduced. That is, because the end clutch C3 receives hydraulic pressure from the release side chamber h1, a drop in pressure to the latter results in a drop in pressure to the end clutch C3.
When the level of hydraulic pressure to the end clutch C3 is reduced as in the above, engine run-up results, wherein engine revolutions are overly increased, such that the rear clutch becomes damaged.
Further, because operational pressure of clutch means operating as input elements in the third and fourth speeds is used for varying line pressure, and as the shift timing control solenoid valve controlling the shift timing control valve, which supplies hydraulic pressure to the clutch means, is always controlled to an ON state in the fourth speed, line pressure from the hydraulic pump is continuously emitted through the exhaust port of the shift timing control solenoid valve in the fourth speed such that the level of pressure is reduced.